城堡周围的高墙和塔楼是它主要的防御工事。

The high walls and towers around the castle serve as its main ___fortifications.

城墙的加固工作是为了抵御外敌入侵。

The ___fortification of the city walls was done to defend against enemy attacks.

牛奶中的维生素D是通过强化添加的。

The vitamin D in milk comes from ___fortification during processing.